The present invention relates to a buttonhole sewing machine capable of forming a buttonhole; more particularly, it is concerned with a sewing machine capable of automatically producing a buttonhole which is formed of a plurality of stitching parts including spaced-apart parallel rows of stitches located on right and left sides of a centerline of the buttonhole pattern.
There have been proposed various types of sewing machines for automatically sewing a buttonhole which have right and left side stitching parts in spaced-apart parallel rows and another stitching part connecting those parallel spaced-apart stitching parts at one end thereof. It has also been proposed, in the art of buttonhole formation, to reduce to an irreducible minimum extent a difference in feed distance of a work material or fabric when the fabric is fed in opposite directions, forward and reverse directions for sewing the right and left side stitching parts, that is, to establish the identical feeding conditions of a work material for both right and left side stitching parts. An example of such technique is disclosed in U.S. Pat. No. 4,159,685. While the technique as disclosed therein is effective in forming a buttonhole with an improved appearance, it is recognized that the formation of a buttonhole is influenced by various factors other than the feeding direction of the work material. Such other factors include a variation in detected positions of front and rear ends of each side stitching part which are sensed by a detector switch in cooperation with a buttonhole presser foot. In a buttonhole sewing with such arangement, however, it is generally unavoidable that positions at which the position detector switch is actuated at said front and rear ends on the work material, are varied more or less due to shrinkage of work material and an overrun of the buttonhole presser foot device. This variation in the actuating position of the detector will cause a variation or difference in length between the right and left side stitching parts which define a substantial length of the buttonhole. As a result, it has been found that said another stitching part is separated from the end of one of the two side stitching parts while it is suitably contiguous or connected to the other. This disadvantage is apparent particularly when the work material is fed at a high rate for increased sewing efficiency, and may result in spoiling the appearance of the buttonhole and consequently degrading the quality and value of the sewn products.
Further, in the art of sewing a buttonhole such as an eyelet-end buttonhole wherein the right and left side stitching parts are connected or contiguous at one end thereof to a circular stitching part, another drawback has been experienced in addition to the difference in length between the two spaced-apart side stitching parts. More specifically stated, for eliminating a difference in the feed distance of the work material due to different feeding directions, it has been attempted to divide a circular stitching part into two right and left halves and sew the two individual halves independently of each other, rather than continuously form the entire portion of the circular stitching part in the same circumferential direction. With this sewing method of separate formation of the two semicircular halves of a circular stitching part, it is generally unavoidable that the two semicircular stitching parts undergo a slight shrinkage due to tensile forces of threads upon formation of the stitches, whereby there is easily devloped a gap or spacial separation at the interface or connection of the two semicircular stitching parts which are disposed in the respective right and left areas on opposite sides of the longitudinal centerline of the buttonhole pattern. The gap thus developed will degrade the appearance of the buttonhole.